QUESTION
How are latex bonding agents for stucco applications
tested? —via e-mail
ANSWER
ASTM C932, Standard Specification for Surface Applied
Bonding Compounds for Exterior Plastering,
is used for comparing “exterior surface applied
bonding compounds for improving the adhesion of cementitious material to conrete or other masonry surfaces or any structurally sound surfaces.” (I’m told that the
ancestor to this specification is an ancient military spec; however,
I was unable to find a copy for comparison purposes, and no
one I talked to in the federal government could find it either.)
C932 describes the required characteristics and test methods for
determining whether a product meets certain performance
requirements. The testing method in the standard explains how
to prepare briquettes made of mortar, apply the bonding agent
to the briquettes, apply fresh mortar over the coated surface and
see whether the bond will withstand 150 psi under several conditions.
Several
different latex polymers are used in the manufacture of
these bonding agents, and as long as they perform according
to the above mentioned testing procedures, they are considered
acceptable. However, I happened to be sitting in on the ASTM
task group meeting that monitors C932, and it came to light
that one of the less expensive resins used in some of these bonding
agents, polyvinyl acetate, has shown some failures that the
testing in C932, as currently written, did not predict. During
a presentation to the task group, Douglas “Gerry” Walters, a
chemist who has formulated such resins, explained that PVA is
alkaline sensitive and can fail in such applications due to
“hydrolyzing.” Walters’s presentation prompted a proposed
change in the test method that would better simulate a wet condition
where hydrolyzing might occur, and that the wet condition
be tested at 100 psi.
Hydrolyzing occurs when an alkaline sensitive latex material is Lee G. Jones is AWCI’s director of technical services. In other words, wet portland cement stucco applied over a PVA based bonding agent may have sufficient alkalinity and moisture to dissolve the film, especially if the substrate was already wet before
the application of the layer of stucco.
Walters explains that a couple of other compounds vinyl acetate
ethylene, vinyl ester of versatic acid or vinyl acetate with acrylic
modifiers are not so alkaline sensitive, and are a better choice
for bonding agents.
QUESTION
I am restoring a wall that appears to be plaster.
I have scraped off almost all the old layers of
paint with a manual scraper. Some paint will
not come off. Should I use belt sander on the whole wall to get
it smooth?
ANSWER
A belt sander might be a bit labor intensive on
such a large surface, not to mention annoying
to the neighbors. I recommend using a chemical
paint stripper once the scraper becomes ineffective. There are many different products on the market. I prefer
the paste type. You can cover some such products with plastic
to keep them from evaporating too quickly so they penetrate
farther into the old paint, which makes for fewer applications.
Once the paint is removed, scrub it down with whatever the
paint stripper label recommends to neutralize the chemical, lest
it cause discoloration or adhesion problems later. Once the surface
is clean (and before patching), prime the plaster with whatever
is compatible with the desired finish. Patch the dings with
spackling compound or patching plaster. All purpose joint
compound is not intended to be used thusly; it is designed to
stick to itself or bare gypsum board. Sand first with 80 grit, then
100 grit. Remove all sanding dust with a damp sponge. Spot
prime the patches with the primer, and finish as desired.
About the Author
Send your
subjected to high alkalinity in the presence of moisture. In other questions to jones@awci.org.
